1. Field of the Invention
This invention relates to a solventless, screen printable, heat curable, conductive ink composition.
The invention also relates to a process for forming resistors or conductors on substrates to form circuitboards and electronic membrane keyboards.
In order to obtain a commercially viable, solventless, screen printable, heat curable conductive ink, several parameters have to be met, to wit:
(1) The ink must be screen printable, i.e., have the right viscosity and other rheological properties such as thixotropy.
(2) The ink, when cured, must adhere well to the substrate.
(3) The cured conductive ink must have sufficient flexibility for use on flexible substrates such as Mylar.
(4) The cured ink must conduct electrically.
(5) The ink should be fast curing, i.e., in less than 30 minutes.
(6) The ink should also cure at low temperatures, i.e., below about 200.degree. C., since above said temperature many of the substrates are distorted.
(7) The conductive ink composition should have a good shelf life.
2. Description of the Prior Art
Conductive coatings are known in the art.
U.S. Pat. No. 3,412,043 teaches an electrically conductive resinous composition consisting essentially of silver flake, resinous binder and finally divided inert filler in specified weight ratios. Therein one resinous binder is an epoxy resin system which is cured by the addition of an amine curing agent at slightly elevated temperatures.
U.S. Pat. No. 3,746,662 teaches electrically conductive coatings comprising certain epoxy resins, particles of tough polymer having carboxy, hydroxy, amino or isocyanate substituents which cocure with the epoxy resin, finely divided metal particles and a curing agent for the epoxy resin. The curing is obtained by heating the composition at temperatures of 125.degree. C. or higher.
U.S. Pat. No. 3,968,056 teaches a radiation curable ink comprising a particulated electrically conductive metal containing material in combination with an organic resin binder which is converted to a conductive coating on the surface of a substrate by exposure to either actinic or ionizing radiation.
Re No. 30,274 teaches a circuit board for activating high voltage flashlamps, said board including a non-conductive, thermoplastic substrate having a patterned electrically conductive coating on one of its surfaces and defining electrical circuitry for the flashlamps, said coating comprising an organic resin matrix curable by UV radiation and a particulated electrically conductive material selected from the group consisting of a particulated electrically conductive metal and a particulated electrically conductive metal containing material, including mixtures thereof with no more than up to about 15% by weight of said particulated electrically conductive material having an aspect ratio of diameter to thickness of a value greater than 20.
It is also known from U.S. Pat. No. 4,020,233 to form cured polythioesters by admixing a composition comprising an ethylenically unsaturated compound containing at least two unsaturated carbon-to-carbon bonds per molecule, a polythiol containing at least two thiol groups per molecule, a photoinitiator and a catalytic amount of a pinacol and, thereafter, subjecting the admixture to UV radiation and heat.
It is further known from U.S. Pat. No. 3,662,023 to form a cured polythioether from a composition consisting essentially of an allylic terminated polyene, a polythiol and a chemical free radical generating agent such as an organic peroxide.
Additionally, from U.S. Pat. No. 4,008,341, it is known to form a cured polythioether from a composition curable in the presence of a free radical generating agent, e.g., a peroxide comprising an acrylate terminated polyene and a polythiol.